Physiological Demands and Contributors to Variance of Core Temperature in 162 Recreational Runners during Distance...

To determine the physiological demands and their associations with body core temperature (Tc) of tropical recreational runners during mass participation distance running in a warm-humid environment (Dry bulb Temperature: 27.2±0.4°C, relative humidity: 87±2%).

162 individuals participated in a 21km (n=84) or 10km (n=78) race. Participants demographics were recorded in a pre-race questionnaire, and in-race measurements of environmental conditions, heart rate (HR, n=115), Tc (n=102), four-site skin temperature (Tsk, n=34) and fluid balance (n=36) were assessed. Real-time monitoring of HR, Tc and geolocation was conducted via a multi-user dashboard. Race split and finishing times were extracted from official results. Correlation and multiple linear regression analyses were performed between various parameters and peak Tc.

Participants achieved peak HR (21km: 183±9(154-209)bpm, 10km: 180±10(156-204)bpm), Tc (21km: 39.4±0.6(38.3-40.8)°C, 10km: 39.3±0.6(38.2-41.1)°C) and Tsk (21km: 34.0±0.6(32.8-35.4) °C, 10km: 33.7±0.8(31.9-35.3) °C). Mean Tc was higher (p<0.05) in 21km (38.7±0.5(37.9-39.8)°C) than in 10km (38.5±0.4(37.2-39.9)°C). Tsk exhibited an 'inverted-U' profile in the 21km but plateaued in the 10km race. Body mass loss was -2.5±1.1(-5.5 to -0.7)% and -1.3±0.7(-2.4 to +0.5)% for the 21km and 10km participants, respectively. Starting Tc (18%), mean HR (13%), Body Surface Area (11%), and average speed (9%), but not age, estimated maximal aerobic capacity nor finishing time, to peak Tc.

Recreational runners experienced high cardiovascular and thermal demands. We observed an 'inverted-U' Tsk profile in the 21km race in contrast to a plateau commonly described in laboratory-based findings. Starting Tc, mean HR, Body Surface Area, and average speed were independently associated with inter-individual differences in peak Tc. Real-time monitoring and contributors of peak Tc may inform future development of targeted strategies to optimise safety of recreational populations competing in the heat.